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What type of airfoil do low-supersonic aircraft use?
I understand that supersonic airfoils have better performance supersonically, but it seems at if they are not always used for any aircraft that goes supersonic and that supercritical airfoils are often used instead. What Mach number or other quality determines whether a plane will go with supersonic or supercritical airfoils?

Related: what is the pressure distribution around a supersonic wing

Designing an aircraft to travel at mach 1 and beyond presents a whole host of challenges that must be overcome. For example, beyond the obvious issue of airfoils, turbojet engines generall need to solve the problem of supersonic intake - slowing the air down as it enters the engine to speeds below mach 1 before compression and ignition. The considerations involved in supersonic flight can greatly increase the design and manufacturing cost of an aircraft.
Therefore, to get the most speed and performance out of an aircraft while still keeping it subsonic, designers will opt for a supercritical wing. Normally, air passing over the airfoil of a wing reaches supersonic speed before the aircraft itself does. To minimize this, supercritical wings are designed with a flat top and a lower aft curvature to reduce the size of the shockwave and reduce drag on the aircraft when flown at near supersonic speeds.
If an aircraft is designed to spend most of it's time in cruise at near supersonic flight, designers will often opt to use a supercritical wing to optimize this characteristic - even if the plane is capable of supersonic flight. While this airfoil shape may not be the best design for prolonged, efficient supersonic flight, its shockwave reducing characteristics still provide some benefit over airfoils designed for normal subsonic flight. They are also much more stable at slower speeds than dedicated supersonic wings.

Thanks for the answer, so would that mean that fighter jets use supersonic or supercritical?

There are certainly exceptions, but most modern fighters are designed to operate well beyond Mach 1, and many use some variant of a delta wing design, optimized for supersonic flight. Interestingly, the F-14 Tomcat uses a NASA 6 series supercritical airfoil. It is a swing-wing system that takes advantage of improved supercritical flight characteristics at slow speed, and can draw the wings back into an aggressively swept (almost delta-like) profile for supersonic flight.

It's NACA 6-series – and no, those are not supercritical.

@Bubinga: Neither supersonic nor supercritical. They use quite regular, but very thin sections.

@PeterKämpf: Here is a link to a NASA technical memorandum. On page 4 it says that “the basic F-14 airfoil shape (a modified NASA 6 series) was used...” The 6-series in question was an early supercritical airfoil. I’ll include a link to that NASA memorandum as well. Here is the first link: nasa.gov/centers/dryden/pdf/88125main_H-1461.pdf. Here is the second link: ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19900007394.pdf

@AaronHolmes please don't be put off by another user's "high rep" and snarky comments - they don't speak for the community. Your input into this site is just as important as anyone else's. Thanks for your answer(s)/contribution(s).

@Jamiec So it is no longer acceptable to point out factual errors? The answer is wrong which can even be proven by the linked NASA paper in the author's own comment. Is that another Monica moment here?

@PeterKämpf: The supercritical airfoil being discussed is a NASA-modified version, based on the NACA 6-series airfoil. The NASA version has substantially reduced curvature of the  mid-chord region of the upper surface combined with increased camber near the trailing edge. Which, you will notice, fits the profile of supercritical airfoil design. The NASA supercritical wing characteristics are compared to the NACA 6 to note the performance differences that the changes created.           https://crgis.ndc.nasa.gov/crgis/images/5/59/Review_SC_Airfoils.pdf

@AaronHolmes: Do you ever read what you link to? The author of the linked paper is at pains to stress that the supercritical airfoil is not a 6-series airfoil. And being Richard Whitcomb, the author should know the difference between those airfoils. Also, the text says that the airfoils were tested on three airplane types but never intended to be used in production versions.

@PeterKämpf It has never been acceptable to phrase it in the way you did. Soften your tone, as you have been asked to do on numerous occasions now. You're a massively smart guy, we all know that but keep it friendly please.

@PeterKämpf: I do read what I post. I said that it was a modified 6 the entire time. Whitcomb writes: “the supercritical airfoil is structurally equivalent to a NACA 6-series airfoil of a somewhat greater thickness-to-chord ratio.” I do apologize, however. I said that I wouldn’t get drawn into an argument, but I continued trying to get the point across. Whether this satisfies you or not, I wish you nothing but the best. Time to move on.

@Jamiec "massively smart"? When I cannot even get a simple point across? I don't know.

Normally, supersonic fighter wings for which airfoil information is published use a very thin NACA 6-series section with very little camber, such as
Aircraft                root airfoil      tip airfoil
McDonnell Douglas F-15  NACA 64A006.6     NACA 64A203
General Dynamics F-16   NACA 64A204       NACA 64A204
Lockheed-Martin F-22    NACA 64A?05.92    NACA 64A?04.29

This information is from The Incomplete Guide to Airfoil Usage by Dave Lednicer. As you can see, the F-16 uses camber throughout, indicated by the 0.2 design lift coefficient of the airfoil, while the F-15 uses an uncambered root airfoil.
Those airfoils are chosen because flow around them at supersonic flight speed is still heavily influenced by subsonic flow characteristics* as long as wing sweep allows for a subsonic leading edge. This produces nose suction which is very helpful to keep drag down. Delta wing aircraft enhance this nose suction by cambering the outboard leading edge (conical camber).
Common to all supersonic wings is their minimization of wing thickness, because wave drag will grow with the square of thickness. Typical sections use a relative thickness between 4% and 6%. In case of the F15, thickness varies between 6.6% at the root and 3% at the tip.
Supercritical airfoils work best in a narrow Mach and angle of attack range and are used for transsonic designs (flight Mach number between 0.7 and 1.0). Fighter aircraft with their wide variety of speeds and angles of attack will rarely profit from using them. The strongly negative pitching moment of supercritical airfoils (resulting from rear loading) makes them unsuitable for most fighter designs.
Supersonic airfoils are reserved for designs with a supersonic leading edge, like the F-104 or X-15 wings and tail or the XB-70 canard. They are only used if top speed exceeds Mach 2.2 and the sweep necessary for a subsonic leading edge would be too high for decent subsonic performance.

* For those with a severe problem of reading comprehension: Of course the flow over a swept wing is supersonic when the airplane moves at supersonic speed. Only the behavior of the flow is like that at subsonic speed as long as LE sweep is higher than the Mach cone angle.

Thank you for the thorough answer! Weird followup question: For experimental unmanned aircraft that are just barely trying to break the sound barrier, would supercritical or supersonic work better?

@Bubinga: as transonic is defined as Mach .72 to Mach 1, a supercritical wing design would be perfectly suited to “just barely” reach the speed of sound while providing additional stability to your UAV in the preceding accelerative phase of flight.

@Bubinga: A supersonic airfoil would certainly be a poor choice. Wing sweep should keep flow patterns subsonic even at supersonic speed. Whether a supercritical airfoil works better depends on the exact specifications – what top speed is required, what speeds are flown for how much time of the mission, at what altitude and lift coefficient. This cannot be answered without knowing more of what is required.


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Vintage army Kollsman altimeter - any info?
I have a vintage Kollsman altimeter A.C. U.S. Army. I can't read the text on the front because of the needle arms are in the way, but might state:
Type 5
Serial No A 464.
Back plate is very hard to read, but does state:
Kollsman Brooklyn NY.
Then possibly: Order No.  W. 636 AC 4708  or W. 635 AC 4798
Any info on it or what aircraft it came from?

This type of altimeter could have been used on a variety of different airplanes.  The only way you may figure out what plane it came from is the serial number if the records are still around.  You may want to contact an aircraft historian if you really want to find out.

You should think about selling that altimeter to a museum because you have a piece of history. Kollsman was one of the first people to make barometric altimeters and a Kollsman was used on the first deomonstrated instrument flight by Jimmy Doolittle.
Excerpt from Wikipedia:

Paul Kollsman (February 22, 1900 in Germany – September 26, 1982 in Beverly Hills, California) was a German-American inventor. He invented barometers and instruments for instrument flight in airplanes.
Kollsman studied civil engineering in Stuttgart and Munich (Technische Universität München). In 1923 he emigrated from Germany to the United States. He worked as a truck driver until he found a position at Pioneer Instruments Co. in Brooklyn, New York. In 1928 he founded his own company, Kollsman Instruments Co., with $500 of seed money.
He spent a long time searching for the right opportunity to launch his product, then Jimmy Doolittle flight-tested his instruments. His instruments were later used in the NASA Apollo program. The altimeter setting window of the sensitive aircraft altimeter is named the "Kollsman window" after him.

I don't mean to be a buzzkill, but it's a bit of a leap to say that this particular altimeter is a unique museum piece just because the founder of the company is important (just like not every Ford car is a museum piece even though Henry Ford revolutionised the car industry). Although of course if it's actually the 464th altimeter produced by Kollman, it might be a different story.

@Sanchises The value of any item is not determined by age, function or even rarity. It is determined by what someone would pay for the item. I'd say that saying this is a good candidate as a museum piece is a fairly safe leap.

Gentleman, Check out the Smithsonian. Many of Kollsman's instruments and those of his engineers, including my grandfather Victor Carbonara, are in the Smithsonian Archives.

source: Stack Exchange

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What is this 4-propeller plane?
My dad noticed it flying quite low today, sadly we do not have a better photo.
4-propellers and uniquely shaped tail assembly.
Plane was spotted in northern Poland, around 9.30AM, near border with Russia/Kaliningrad.

Have a look at my answer - and especially the many images of each type of craft. I do not know the exact location but (see map in my answer) given the appearance and the various locations mentioned, the IL38 appears the more likely choice. I may be wrong, of course :-).

See the map in my answer - can you identify the location at which the photo was taken and the direction that the photographer was facing.

Where you spotted it would help with probabilities.
However - the tail marks it out as a Maritime Patrol aircraft, of which the only low-wing 4-engine one is the P-3 Orion operated primarily by the US and Oz.

Added time & location details to question, thank you.

There's one other low-wing 4 engine maratime patrol aircraft, the Ilyushin Il-38. Although indeed this looks more like the P-3 Orion.

I think your answer would benefit from some information as to what exactly about the tail makes it obviously a maritime patrol aircraft.

@AlexanderKosubek Agreed.  The MAD boom on the tail is there specifically for submarine hunting.

For interest: I'd have suggested a P3 - New Zealand (where I live) also operates them although I believe they are about to be phased out.

@Sanchises See my answer.

How did you know that it was a maritime patrol aircraft based on the tail?

@dalearn: The MAD sticking out of the tail.

The fuselage extension at the back of the empennage is similar to that of a Lockheed P-3 Orion

While I personally would have thought it was an Orion P3 (as it resembles the Orions flown by the NZ airforce), I suggest that Sanchises suggestion of an Ilyushin IL38 (not II38) seems more likely. This is based on numerous online images of both craft 'at a distance' and the location. The wing shape appears (to me) more like that of the IL38, and one could 'pretend' (given the small image size) that the outer engines appeared somewhat smaller, as is the case with IL38's 'at a distance'.
The IL38 is or was operated by USSR/Russia and India.
Depending on how close the location was to the Russian border the aircraft may have been in Russian or Russian-Kaliningrad airspace. The latter would seem to be an entirely likely location for such an aircraft.
It is also entirely possible (and I have no knowledge either way) that Russia has overflight rights for military aircraft between Russia proper and the Kaliningrad exclave.

Apart from the US Navy (whose aircraft may turn up almost anywhere :-) ) none of the 18 operators of the P-3 are anywhere near the location noted. .
Many P3 Orion photos here
Many Ilyushin IL38 photos here
Here is an image of a US Navy P3 Orion.
It will be seen in many of the images linked above that from a distance the wings tend to appear slightly swept forward due to the larger rear root to tip taper compared to the leading edge taper.

________________________________
Here is a diagram of an IL-38
Looking at the many IL38 images linked above shows the wing to appear at a distance as thinner and squarer ended than a P3 Orion.

I don't see the radar bulge though, which I think should show up even on the low quality picture.

@Sanchises In photos, not all have them, and some have what look like garden sheds on top :-). I looked for other feature differences as well, but could not convince myself with any degree of certainty that the RADAR dome was missing as opposed to masked. Knowing the photographers location and line of bearing may help.

Among the P-3 opeartors, the Netherlands, Norway, Portugal and Spain  are all in NATO, so could reasonably be flying near Kaliningrad. Amsterdam and Oslo are closer to Kaliningrad than Moscow is.

@DavidRicherby the Netherlands haven't operated the P-3 for over a decade. The aircraft were given to Germany (for free).

@jwenting Thanks. I just followed the link in the answer, which said "operators", implying current. I've edited the answer to point to Wikipedia instead. Russell, could you please edit your claim about none of the operators except the USN being close? Germany is right next door. Germany, Greece, Norway, Portugal and Spain are all in NATO so could be flying over Poland. Berlin and Oslo are closer to Kaliningrad than Moscow is, and Athens is only about 50km farther away.

Also, the aircraft was described as flying low so, unless it appeared very close to the border, it was probably on the Polish side. It's not like it was at 30,000ft and was 20 miles from the observer. And even if Russia does have overflight rights to reach Kaliningrad, they'd be over Lithuania and Belarus, not Poland. That seems unlikely anyway, since Lithuania is in NATO and Kaliningrad is accessible via the international waters of the Baltic.

My dad reviewed all the answers, he says the plane certainly looked more like P3 Orion rather than IL-38 all things considered. It was about ~10km from Vistula Bay shores. Sadly, we have no clue who could possibly be the operator of the plane.

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If a Boeing 707 hits turbulence that registers as +5 G on the FDR, would the 787 experience lower G forces due to the flexibility of their wings?
A 707 being a much older plane with relatively stiff wings compared to the 787. What percentage would the g forces be reduced on a 787 given its wing flex?
And yes, I mean if the 707 and 787 were flying through the exact same turbulence at the same time.

The B787 has a gust alleviation system that likely has a much greater impact than the wing flexibility.  Related: Does the 787 gust alleviation system make a more comfortable ride or for a more stable aircraft?

@Gerry is there any data on what % turbulence g factor would be reduced? Ie if it went through the same turbulence as the hypothetical 707 in my post title.

I expect that Boeing's flight controls group may have access to the raw data needed to make the comparison. That doesn't mean they've actually compared the two platforms. It's also unlikely that any significant data has been published outside Boeing as it would be considered proprietary.

Gust is an impulse load on an aircraft: a sudden change in force, which distorts the previous equilibrium. Upon a vertical gust, the aircraft will respond with a second order response: vertical acceleration, damped by the air resistance.
The factors for accelerations due to gusts of a certain intensity and duration, as per this answer:

Elasticity of the wing construction material acts like a spring. Load it with a vertical gust, and it will bend upwards, then spring back.
The wing bending experiences damping forces from the surrounding air, proportional to bending velocity.
The wing sweep angle spreads out the gust: not all of the wing is accelerated upwards immediately.

When comparing the two planes (from the wiki pages):

Faster planes are less impacted by a gust load - the B707 cruised at 1,000 km/h, the 787 at 900 km/h, so this is in favour of the 707.
Wing sweep is comparable, 35° for the 707 and 32.2° for the 787
Higher wing loading is favourable: 151,300/283 = 535kg/m$^2$ for the 707-320;  254,011/377 = 674 kg/m$^2$ for the 787-10, the 787 wins
Higher aspect ratio is favourable: A = 7 for the 707-320, 9.59 for the 787: it wins

Simple observation as a passenger reveals that the carbon-fibre composite wing of the B787 flexes up-and-down a lot, so the spring-mass-damper response of the B787 would be slower, despite its 10% lower cruise speed.
Screenshot from this video
The active gust alleviation system of the B787 deflects the inboard flaperons as a function of detected vertical acceleration, which indeed helps in comforting out the ride.

The 787 will not experience lower G-forces, but it will smooth out short spikes. If a 707 experiences +5G for 200ms, a 787 might ride over it and not feel much. If a 707 experiences +5G for 2s, so will the 787.

What percentage would the g forces be reduced on a 787 given its wing flex?

You cannot give a percentage of reduction. If the updraft persists for long enough, both aircraft will feel the same G-forces. If anything, you can give a maximum gust duration that can be smoothed out.

Since G-force is an acceleration, I do not think this answer can be right. The root of the wing will start to produce lift instantaneously on both aircraft. On a 707 the tip will also produce lift very quickly. on a 787 the flex of the wing will distribute the force output over time, thus expanding the exertion of the force over a longer period -> Z less G-force.

@Jpe61 - Your comment is correct, but I cannot see where it disagrees with my answer or what point you are trying to make. A floppy wing does not reduce G-forces in general, but it does average out short spikes. That's what my answer says and that's what your comment says. OP was asking for a "percentage of reduction" and that doesn't exist. If anything, you can state the "maximum gust duration" that will be averaged out (longer on a 787 than on a 707). Longer gusts will always produce full G-forces, on both aircraft.

Gust by definition is a brief change in wind. During this brief period the gust will load the wing of 787 gradually bwcause of the elasticity, so the effect of the gust force is spread over a longer period of time resulting in lower vertical acceleration of the airframe (m/s^2). Think of it this way: you have two equal weights on table, attached to the edge of the table, one with a rubber band, one with a chain, both of equal length. Now push both over the edge of the table. Which one of the weights will experience higher g-forces as comes to a stop?

@RainerP. You are conflating the cumulative force delivered to the aircraft by a gust with the number of Gs that the aircraft experiences.  If I experience 2Gs for 3 seconds, or 3Gs for 2 seconds, it may seem like I have experienced the same amount of cumulative force.  But in the first example I experienced 2 Gs, and in the second I experienced 3 Gs.

G-forces caused by turbulence are instantaneous. The wings will not "cushion" the aircraft to mitigate the force of the turbulence. The additional flexibility of the wings means that they will absorb more energy and dissipate them without damage to the structure.


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Why are the MSA direction needles not oriented to north?
Why is it when you look at an MSA the arrows are pointed opposite of where they should be?

The arrows are referencing the nav cues, not the VORTAC radials, which define the MSA zones.  It’s cut up that way to define the sectors relative to the reference point that you will be flying in e.g. for the second from left MSA symbol, if you’re flying southeast of the HQM VORTAC between a course heading of 230° TO and 350° TO the VORTAC, this is an acceptable MSA for that segment.  I suspect that choice was made because aircrews are going to be approaching the airport from these areas as opposed to departing and their HSIs, OBSs, RMIs, etc are going to be set to track TO the NAVAID at that point.


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Are you required to fly a full hold when doing a course reversal?
When doing a hold in leu of a procedure turn are you required to do a full hold for the course reversal or just the entry to the Fix and then proceeding in bound for the final approach course?

Welcome to aviation.SE! Are you asking about US/FAA procedures? For questions about regulations, procedures etc. please always tell us which country or regulations you're interested in.

No, you're not required to do a full oval-pattern hold. Once you're tracking the inbound course immediately after the entry maneuver (teardrop/parallel/direct) and pass the holding fix, you're done and can proceed inbound on the approach.  I do want to clarify that even if it is a perfectly direct entry, one turn around the hold is still expected.
Either the teardrop, or parallel entry turn-around is adequate and then once you cross the fix upon which the hold is based, you may continue straight inbound on the remainder of the procedure.
Of course, any ATC instructions to remain in a holding pattern override this default.

When performing a HILPT, typically the aircraft will pass the hold point outbound and perform the correct entry.  When you are inbound, you will continue with the approach once crossing the hold point again.
The aircraft will only pass the hold point twice unless a request is made to ATC for further hold to get ready to fly the approach.

In the Unites States, the FAA has this to say about what they call an "hold-in-lieu-of-PT" (HILPT) in AIM 5-4-9(a)(5):

The holding pattern maneuver is completed when the aircraft is established on the inbound course after executing the appropriate entry. If cleared for the approach prior to returning to the holding fix, and the aircraft is at the prescribed altitude, additional circuits of the holding pattern are not necessary nor expected by ATC.

So no, a full hold is not required; just the entry and then getting established back inbound to the holding fix.


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What are some examples of holding?
What are some examples of holding instructions and how to enter them? I understand entry with direct, teardrop, and parallel. Just having a hard time coming up with formulating the procedure. Example; C172 hold south of ABC VORTAC. Does that mean race track is the 360 radial and holding south of the vortac?

FYI, from my experience you will always hold at some fix as depicted.  I have never had ATC just assign me a random hold somewhere.  There are plenty of charted holds to choose from.

Closely related, maybe a dupe?

Remember, radials come from a fix.  You cannot hold south of a VOR on the 360 radial in the same way that you could not hold east on a 270 radial using the VOR as the holding fix.
You could hold south on a 360 radial, but the fix would have to be somewhere away from the VOR.  For example, if you're told to hold south of the 10DME on the 360 radial, that could be done.
What that would mean is that your fix, for the purposes of the inbound course, would be the 360 radial of the VOR at the 10 DME distance.  So your racetrack pattern would be south of the point 10 miles north of the VOR (per the compass rose's north).
Examples:

Spot on, Ryan. You may want to clarify that the fixes that have radials originating from them are VORs. You can have a non-VOR fix located along a radial of a VOR identified by the DME distance from the VOR, or triangulating with a second VOR. Any directions from or to a non-VOR fix would be called a bearing. A local DPE in this area has a trick question on the checkride where he gives holds either based on whether he says radial (from the Maverick, Ranger or Cowboy VOR) or bearing (from the fix). His reasoning? “Words mean something. Use the correct ones.”

A holding instruction will usually include:

A fix,
A bearing or radial on which to travel for your inbound leg,
A turn instruction (if the turn is not the standard right turn)
A time or distance to travel on your inbound leg.
A cardinal direction of disambiguation for redundancy to keep you on the correct side of the fix.
An Expect Further Clearance time.

In GA flight, I have never received a non-published hold from ATC. Although, I have requested and received clearance plenty of non-published holds during training as well as my checkride.
Research AIM 5-3-8 for further details.

Non published hold clearances are usually for holds at some fix requested by the pilot to deal with a situation.  Without an FMS, or a GA GPS system with FMS features that let you set up holds at any waypoint, almost unheard of.  If a controller wants you to hold somewhere at a random fix for his/her own purpose, they will give you lots of advance warning to be ready to write down the clearance.

@JohnK - Agreed. Though, in training (and on my Checkride), non-published holds we’re done without an FMS or GPS (deemed inop by CFII and DPE for demonstration purposes). We did non-published holds with either two VORs determining the waypoint or one VOR and DME determining the waypoint. Each time, the holds were requested from ATC mid-flight along our IFR route. Usually it was done right before canceling the IFR clearance in order to divert to a nearby IAP for a surprise (to me) approach.

Yeah. I had to do holds at an ADF fix!  Where you have to guess the drift correction, fly with the needle offset, and you know you're good because the offset needle doesn't move when inbound.  All kinds of stuff done in training that you'll never see in the real world, at least in this day and age.  The workload on that sort of thing is pretty high, and I would say, dangerous when single pilot.

@JohnK - By the way, take a look at the other hold question. Am I missing something here? Or, is the OP way off?  I haven’t done a hold since COVID-19. https://aviation.stackexchange.com/questions/79852/holding-pattern-direction

Published holding us easier for both the pilot and ATC, but sometimes impossible due to a weather system impacting arrival routes. Plenty of non-standard holding is issued as published patterns are covered by convective activity.

Welcome to [aviation.se]. The original question is a bit unclear to me, and I'm really unclear as to how this answers it. Please [edit] your question to provide some more details. If you'll take the [tour] you'll get a feel for how this site differs from "regular" discussion forums, and if you'll read the [help] on "answering" you'll get a pretty good idea of the kind of authoritative and detailed answers we're looking for here.


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What happens when Johnny Troublemaker is kicked off of a flight?
We've all seen the problem passengers that behave so poorly that they have to be deplaned from the flight. I think that most are not criminally prosecuted (but I could be wrong) so my question surrounds what actions the airline(s) take. Specifically, does the airline put Johnny on a "no-fly" list forever, and is this info shared with/enforced by other airlines and/or TSA?

Whoever voted to close this for being "unrelated to aviation" is higher than airborne planes!

There is such a wide range of "it depends on the circumstances" (and the country you're in, and the airline you're on, and the individuals involved) that no single answer is going to always apply.
In the most simple case of a passenger who's had too much to drink and complies when they're told to deplane, it's possible that they have no greater punishment than a few hours to sober up watching airport TV (which is, admittedly, pretty bad), and once they have sobered up, they might be on a later flight.
On the other end of the scale, the utterly noncompliant passenger who decides to fight with law enforcement officers after attacking people on the airplane and damaging the aircraft itself is going to be in a world of trouble.
And everything in between can have outcomes in between.
Me personally, I've had several cases of deplaning intoxicated customers, most of whom left uneventfully. (I was no longer around that airport to know if they got on a later flight or not.) I've also had cases where law enforcement met us in the jetway, and I can think of two offhand where an arrest was a very real possibility. In one case, after what was apparently a pretty lengthy interview, the police decided to let the guy go; in the other, the perp absolutely should have gone to jail, and that only didn't happen because the step-father meeting his kids' flight declined to press charges. My opinion of him was almost as low as my opinion of the offender.
As far as airline no-fly lists, those do exist, but what exactly it takes to get on one is beyond what we as pilots typically see.

The no-fly list was what I was particularly interested in. I was wondering how long an offender would be on it and how does that interrelate with other carriers/TSA.


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Did the Space Shuttle really wait until 50 feet above the ground before they put the gear down?
I saw a sign at the Smithsonian about the Space Shuttle where it said that their gear was not extended until 50 feet above the ground.  I've searched and searched for something definitive that had the actual procedures used, but can't find anything.
To put this in perspective, the policy for every jet that I have flown has been to have the gear down and locked between 1,500 ft. and 500 ft. (at the very lowest).  50 feet seems very very low! What happens if it doesn't come down on the first try and they need time to troubleshoot?

Not very official, but http://www.spaceshuttleguide.com/system/landingdeceleration_system.htm#Gear_Deployment says about 300 feet, not 50.

If you're looking for a definitive source, how about the Shuttle Crew Operations Manual. It's essentially the POH for the Space Shuttle. Secton 2.14-2 says:

The landing gear is deployed at 300 ± 100 feet and at a maximum of 312 knots equivalent airspeed (KEAS).

Although, the targeted deploy speed was 288 KEAS according to the Normal Procedures section 5.4-7, which matches up with approximately what you see in HUD videos.
The manual says the gear may take up to 10 seconds to fully deploy, but based on HUD videos (measuring the time it takes for //GR// to change to GR-DN), this process pretty consistently took about 6 seconds and was completed by about 70-130 feet AGL depending on the deployment altitude.
The reason they deployed so late is because the shuttle was a very non-aerodynamic "airplane" to begin with, and lowering the gear would reduce its lift-to-drag ratio even further. However, there were non-standard procedures in place (see section 7.4-25) where, if the speed brake (a split rudder – not a wing surface) was stuck closed, they would have lowered the gear earlier in order to act as a speed brake. Note that the hydraulic system was incapable of raising the gear in-flight, so once down, it stays down.
Because the shuttle was a "glider" (a generous use of that term) at landing, with no chance of going around, the gear absolutely must work on the first try. A shuttle belly landing would have ended very bad due to its high speed and high angle of attack. In order to assure the gear would extend, there were several redundant systems in place:

The doors had a bungee-assist system which exerted 2000 pounds of force (~9 kN) on the nose wheel doors, and 5000 pounds (~22 kN) on each of the main wheel doors.
The nose wheel had a pyro-assist system which fired every time the gear was deployed, and helped assure it would lock into place.
The gear is normally deployed through a combination of "springs, hydraulic actuators, aerodynamic forces, and gravity."
However, if all else fails, and the gear does not begin to move within 1 second of issuing the command, a pyrotechnic initiator cuts the locks and forces the gear down.

So they were pretty confident it was going to work.
Interestingly, the original procedure for deciding when to deploy the gear was based on airspeed, not altitude. However, this resulted in an inconsistent safety margin. This quote from the Flight Procedures Handbook: Approach, Landing and Rollout explains it better (sorry, I don't have a link to a doc, I probably downloaded it from a nasaspaceflight.com forum a few years ago with a paid membership):

The landing gear deploy cue for STS-1 through STS-4 occurred when the velocity decelerated through 270 KEAS. This corresponded to an altitude of 200 ft on the nominal energy trajectory. Flight STS-1 followed the velocity cue procedure which, due to its much higher than expected energy, did not occur until an altitude of 85 ft. Had the gear been deployed at 200 ft, some of that excess energy would have been dissipated. Flight STS-2 was very low on energy, reaching a maximum velocity of only 274 KEAS at 1100 ft altitude. The 270 KEAS cue occurred at 600 ft, but the actual deploy occurred at 400 ft, adding to the already existing low energy condition. Lower energy occurred since the nominal altitude for gear deploy would have occurred at 200 ft on a nominal trajectory. Flight STS-3 was high on energy, not decelerating through 270 KEAS until an altitude of 87 ft. T/D occurred earlier than expected on STS-3 and the gear was actually down and locked only a couple of seconds before first wheel contact. It was after STS-3 that altitude was selected as the gear deploy cue because it would compensate for off-nominal energy conditions, not make them worse, and still satisfy safety concerns. Downrange was also considered as a gear deploy cue, and it too, had advantages over velocity.

As far as the sign you are referring to, I see three possibilities:

They were referring to the very early missions which sometimes saw very late gear deployment,
They were rounding down for dramatic effect, or
They simply made it up on the spot as if it were a statistic.

Maybe they should have said that it was not fully extended until 50 ft.  That would have been closer anyway.

I always thought there was something definitely off about the nose gear on the shuttle.. like it was too short maybe. The thing seemed to be terribly unstable as it went horizontal and the nose gear hits VERY hard on some landings.

@Trevor the nose gear probably hits a bit harder than most airplanes, but it's really not that hard. STS-3 definitely hit hard. The nose gear strut was shorter to save space and weight. However, I wouldn't call it unstable. There was really no reason why it needed to be perfectly horizontal. It's not like it's a passenger plane where the tilt would make it awkward to load/unload.

@BretCopeland ya I meant as the wing goes under horizontal in landing the nose gear is still a few feet off the ground. Then it drops like a stone. See about 2:40min on the video here https://www.youtube.com/watch?v=dDvyznX1ipY. Note it over compensates first and noses up...

@Trevor as I said, STS-3 hit hard, but STS-3 was a very off-nominal landing in many ways. It came dangerously close to exceeding the structural limits of the landing gear. So it's not a good example.

Yup..still.. it does go unstable at the rotation point... but too late to fix in the design cycle I guess.

@Trevor maybe I'm not sure what you mean by unstable. Just because it's nose down doesn't make it unstable.

I mean as you pass the rotation point, in order to gently lower the nose you need to input quite a large pull back on the yoke to compensate for the wings driving the nose down... It's VERY touchy and you don't get a lot of practice on the real thing.

A few more feet in the nose gear or shorter main gear would have corrected that.

@Trevor I see your point, but I just don't see it as a very dramatic effect, and I don't think it ever offered any operational disadvantage. I mean, here's a great look at the derotation for STS-128. It's quite smooth all the way to the ground, and that landing even used a late chute deployment (the drag chute normally helped stabilize the derotation, but was deployed late if there was enough of a cross wind). Making the main gear shorter wasn't an option because you'd increase the risk of a tail scrape.

Indeed :) they got a lot better at figuring out the dynamics in the later flights for sure. Still, quite the deflection in the nose gear though, must have been quite a jolt in the cockpit. I do realize there was not a heck of a lot they could change once built. I have to say, from a cosmetic point of view, it just looks wrong too, but that may be just my opinion :) It kind of looks odd, like a car running on one of those small emergency spare wheels... LOL. Either  way, it was, is, one heck of an amazing machine.

I believe the short nose gear is an intentional design. They want the thing glued to the ground once the nose wheel touches. All planes would probably be designed this way for safer landings if it weren’t that they also need to be able to take back off again.....a task the shuttle would never need to do.

@GregTaylor I don't think the design had anything to do with keeping it glued to the pavement. After landing, they put the elevons down (acting like flaps) in order to reduce the weight on the gear. My understanding is that the short nose gear was simply to save weight and perhaps a little bit of space.

Maybe the OP mis-remembered and the sign said 50 meters, that puts the number in feet in the same ballpark as the figures in this answer.

The official source says that the landing gear is deployed at around 1700ft AGL, however I suspect an error in that statement.
According to the document though, the spaceshuttle has a 1.5 degree glideslope in the short final and around 200kt. In this video the landing gear is extended 18 seconds before the touchdown. 1.5° glideslope equals 2.6% glide path (tan 1.5). 200kt are 337.56 ft/s and 2.6% of that are 8.8 ft/s. The space shuttle is therefore descending with about 9ft/s in the very short final. 9*18 equals 162, which means that in this video the gear was extended at roughly 160ft.
However, these are rough estimates. Before the short final the space shuttle has a descent rate of 10,000ft/min, therefore it might be, that it was even higher in the video. But even though if it was 10ft/s, that would mean that for 50ft the gear is not extended five seconds before touchdown, not counting the time it actually takes to extend it into a safe position.
The link Qantas 94 Heavy posted in the comment says "300 ± 100 feet", so it would be somewhat close to that. My guess is, that the spaceshuttle is still faster than 200 kt at this point and therefore has a higher sinkrate.

The gear were lowered manually and could not be retracted, so they were lowered shortly before touchdown. In this video the Columbia gear was lowered just about 8 seconds before touchdown (at Edwards AFB). http://www.youtube.com/watch?v=dDvyznX1ipY

Oh wow, that is impressive. And that is easily 50ft.

Maybe they meant that the gear is not fully extended until 50'....

That video really shows the nose gear instability problem.

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text:
What's the typical cost (and its breakdown) for a long-haul commercial flight?
I realised that this question is very broad, but I intend it as an example to illustrate the actual costs incurred by a typical long-haul commercial flight.
For the purpose of this question, I would assume the following:

A380 or (modern) B747 in typical 3-class configuration
Full occupancy
12 hours flight time (or more)

Here are the cost position I can think of right now:

Fuel
Aircraft amortisation
In-flight crew salaries
Ground crew salaries (boarding crew, aircraft preparation and cleaning, luggage loading, etc.)
Catering
Various insurances
Airport fees (landing tax, etc.)
Overflight fees
Administrative cost (managing/issuing tickets, etc.)

What am I missing? What would be the absolute (and relative) cost budget for each of these?
Bonus question: in airline operations, is this cost typically handled as "per flight hour" (as in general aviation), "per flight", or in another way?

It might be better to look at this as a cost-per-hour (for the actual flight: Fuel, oil, maintenance reserve, crew salaries) plus fixed costs (things like insurance, airport fees, etc.) - It's still going to be hugely variable, but I think the resulting numbers will be more useful.

@voretaq7: such a break-down maybe better indeed, and might even provide a better generalisation. Maybe there are cost items that are per-passenger, like insurances?

@Voretaq7 airlines usually calculate it by "seat-mile." The airline has a certain number of SMs available, and they determine how much revenue each of those need to bring to break even.
This lets them break the costs away from an individual flight and easily see the big picture. Obviously there are many ways to do what OP's asking, but as far as I know that's how the pros look at it.

I think you have most covered but you are missing the Air Navigation Service fees (ATC, MET etc) in the breakdown.

I once saw a documentary about the inner workings of an airline. It said that the airline (for that particular flight and aircraft) only made a profit off of the last few seats. So if the plane wasn't full, they were losing money.

Direct computation of cost per flight would be better. Only part of the costs are variable with flight hours, others are fixed costs (per calendar month). It makes no sense to treat fixed costs as variable costs.

Fuel
The fuel consumption of the A380 is about 11 metric tonnes per hour. With a fuel price of \$1000 per metric tonne, this results in \$11000 per flight hour.
Amortization
An A380 will cost about \$350M. The aircraft will be used for about 25 to 30 years, but let's assume the amortization period is about 20 years with a residual value of \$50M. With an interest rate of 5%, you would pay about \$24M per year.
That is about \$2740 per hour, or \$0.76 per second, flying or not.
Assuming a usage of  60%, taking into account turn around times at the gate and maintenance downtime,  it boils down to \$3915 per flight hour.
Crew
Captain costs \$160K per year, the F/O \$90000. Let's assume they fly about 60 hours per month, the combined flight deck crew cost is about \$350 per flight hour for the base salary. Another \$80 needs to be added for allowances bringing the total to \$430 per flight hour.
Cabin crew rate would be around $40 per hour, so for 20 crew the cost will be \$800 per flight hour.
Also allowances for overnight stays and hotel costs need to be taken into account
Airport charges
These vary extremely between airports. You can basically break the costs down into costs related to the aircraft, the passengers and various taxes by the government.
Aircraft cost usually are related to the Maximum Take-off Mass (MTOM) of the aircraft and the noise category. A Boeing 747-400, while less heavy than an A380, will in some cases be more expensive due to the higher noise level. Many airports differentiate between peak and off-peak hour landings.
I can't find accurate data for the A380 but an old table that I have for the Boeing 747-400 suggest that total the cost vary between about \$6000 to \$25000.
Overflight charges
Fees are levied by countries whose territory is flown over, mainly to cover ATC cost. The cost is typically based on the distance flown and the MTOW. For a 747-400ER over Europe, the fee is on average 145EUR per 100km. Assuming an average speed of about 900km/h, the hourly cost is about 1300EUR/h. These costs are lower over the high seas and over the US.
Rough total
Based on the above, the total will be between \$22,000 and \$41,000 per hour.

This is very interesting, and it shows why some airlines would consider replacing still-flight-worthy B747 and similar craft for B787 and such, as fuel per flight hour could cost double to triple the amortization on the plane itself.

@dotancohen My understanding of Airline Economics is that fuel is by far the dominating cost factor - that's part of why we many airlines have single engine taxi procedures, encourage shutting off the AC packs during extended ground holds, and why companies are looking into things like electric motors on the wheels of airliners for taxi (which could be powered by the APU, using much less fuel than even a single-engine taxi).

Why not develop more robust taxi tow procedures? I've always been amazed at the idea of taxing around with a jet engine. The plane doesn't need to carry around the electric motor, just pay some service at the airport to tow the craft.

@dotancohen The logistics would be incredibly complex with the number of airplanes moving around at the same time, the radio communication and coordination between the people towing, ATC, and the pilots, and then we would need special roads (and intersection procedures) for them to get to and from airplanes at the runway....  It's crazy enough as it is, I can't imagine what it would be like if everyone was being towed!

I see, thank you. To an outside observer, it looks like a simple issue of just having a tow vehicle ready at the end of the runway. As has been said "Everything looks simple to those who don't know what they're talking about".

@abey thanks for adding the overflight charges. I've added that the charges in Europe are higher than in the US, and over international water they are even lower.

Do these costs include taxes? Currently the majority of the price I pay to fly for 12 hours UK to Japan is tax.

I believe that a 12-hour flight will require a relief crew, so you would have to double the flight-deck crew costs. Though, the time spent not flying may be paid at a lower price than time actually strapped into the seat.

No maintenance? Is it negligible?

On the bonus question: the way the costs are looked at by management is cash flow per week. An airline has more than one aircraft, more than 1 flight/day per aircraft, more than 1 crew per aircraft, flights have different durations etc etc. The only way to make sense of it all is to look at operations cost and revenue per day/week/month.
Part of the costs are fixed costs and part of them are variable, and dividing all costs by one of the variables is never a good idea. For quick decision making, treat fixed cost as a given and only look at variable costs. So for instance if we have an aircraft and a crew taxiing on the runway (fixed cost), how do we reduce variable cost such as fuel? By switching off an engine.
Cash flows per time are the only fairly objective way to measure the proceeds of operations. Profit/loss calculations are for accountants, they can be done according to many methodologies. Cash flow is measurable, profit/loss is an opinion.

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text:
Why don't Transport Category aircraft have a listed Vy?
I was talking with a pilot about an inability to maintain the targeted climb rate in a particular situation and asked him what the Vy was for the jet.  He curtly replied "Vy is only for Part 23 aircraft." (FYI for those outside the US:  Part 25 aircraft are commerical planes, while Part 23 are usually general aviation planes. You can find more details here.)
Yet, I know for a fact that there is a maximum excess thrust speed, because at a certain speed drag is at a minimum. This should be close to Vy.  I also know that rate of climb is frequently listed for these aircraft. However, the optimal speed for gaining altitude isn't usually listed for commercial aircraft like it is for GA aircraft.   In fact, many of the v-speeds like Vmca and Vdec are harder to find for larger aircraft.

Why is this?  I can think of several possibilities:

Commercial aircraft speeds are defined by ATC and operating procedures, not by performance considerations
Commercial aircraft have enough thrust that climb performance limits isn't an issue (which I doubt from my experience).
Commercial aircraft use other parameters for similar purposes like rate of climb, v2, or long range cruise speed
Jets don't list this speed but propeller planes do

First things first. All the airplanes have a speed for best climb rate (Vy), and this includes transport category aircraft. But the speed Vy, does not matter to us because our climb performance calculations are a little bit more complex when compared to a small GA aircraft.
Think of this. For a general aviation pilot who flies a small aircraft, the main cost he/ she has to bear while flying the aircraft is what he/ she pays for the fuel. Other costs are miniscule when compared to that. So, the aim of a GA pilot will always be to fly while burning the least amount of fuel. Climbing and Vy gives exactly that as it allows for a much more fuel efficient flight, because the faster you reach the cruise altitude the faster you can reduce the power and lean the engine for the least amount of fuel burn. So, Vy matters in a smaller aircraft.
Now think of a transport category aircraft flown by an airline. Yes, the fuel is still a major cost, but an airline also have to pay for the lease or if they own the aircraft they need to cover the capital, they need to pay the pilots, cabin crew and ground engineers and they also need to put the aircraft away for much more longer, scheduled maintenance to keep it airworthy. All of these costs are what we call time related costs. The relationship between these time related costs and fuel related costs is known as the Cost index (CI).

CI = Time related costs / Fuel related costs

As pilots, we insert a CI into our flight management computer. A higher CI means, you will go faster burning more fuel but in the process, you will save time on the trip and save the airline time costs. On the other hand, flying at a lower cost index will burn less fuel and thus saving on fuel costs, but it increases flight time and the result is an increase in time related costs. Based on the CI and very importantly the weight of the aircraft, the computers will calculate an ECON climb speed called an IAS(ECON). And this is the speed that we use to climb at.
If you want the airplane to climb at roughly the best rate of climb, you can set the cost index value to zero (0) in the flight management system. The computers will then calculate an IAS(ECON) to climb, based on the weight. This speed will be the Vy of the aircraft because at a zero cost index the whole aim of the aircraft will be to get to the cruising altitude faster and reduce the time spent in the fuel consuming climb segment so that more fuel is saved. The CI value will vary based on airlines. Some airlines have a very low CI while others have a higher CI. Most airlines prefer to keep it in an in between value, that is keeping a balance between fuel related and time related costs.

Large commercial jets operate within a much wider range of weights than most GA aircraft do, and since Vy varies with weight, it wouldn't make sense to publish "a" single Vy speed for them. The crew can get a best rate or best angle of climb speed from the FMC (which "knows" the current weight of the aircraft, based on current fuel & an entered Zero Fuel Weight).
From an empty aircraft with minimum fuel to a loaded aircraft with max fuel, the weight probably won't quite double for most commercial jet aircraft, but the one might be 170% or more of the other. With that much range, any single Vy you publish would be way far off in a lot of cases. On a GA aircraft, without an FMC and operating in a much narrower weight range, publishing a single speed makes much more sense as the delta between published and actual becomes smaller and the consequence of being a couple knots off is probably slight.

On the B777 the FMC climb page only lists the Maximum Angle Speed (best angle). We add about 60 knots to get Best Rate.

I'd figure there'd be a QRH/... table for best climb, no?

I think you're right about the FMC, but I don't think the weight variation is much lower in small GA aircraft. A Piper Arrow has a zero fuel weight of 1798 lbs vs MTOW of 2758 lbs, while an A320 Prestige has a zero fuel weight of 137800 and a MTOW of 172850.   So the weight would probably vary  as much in a large jet as a small GA craft and the stated Vy could be just as inaccurate (large ones like the PC-12 are another story).

@CodyP   I suspect that the 137,800 number is a MAX zero fuel weight, not an empty weight. Basic operating weight for an A320 is probably closer to 100,000. Then 150 people at 200# each gives you 130k ZFW, plus bags/freight, and you're close to your max ZFW. Then add 35,000# of fuel, and you have the full jet at max T/O weight. If 137.8 was empty weight you couldn't haul much load very far.

@UnrecognizedFallingObject   There are QRH tables to back up an inop FMC, yes. But the FMC is primary -- way faster than looking up the tables!

FWIW, basic operating weight for old 747-400 freighters that I'm aware of runs from 351088 to 365970. Add 40000 in fuel for a short ferry flight, say KJFK to Dover Air Force Base, and you've got around 400000. Max takeoff for those aircraft runs from 870000 to 910000, so a range of over twice. Those figures are from http://terryliittschwager.com/WB/index.php, scroll down to AIRCRAFT OVERVIEW.

@RalphJ -- yeah, figured the FMC would be quicker than pawing through the QRH xD (although the QRH tables probably are worth mentioning in the answer as they can be used as a reference when you don't have an airplane in front of you :p)

@RalphJ, WP lists OEW of A320 at 93,900 lb, so quite a bit less than 100,000. 137,000 is indeed MZFW.

@MikeSowsun, somewhat surprising to me. As far as I found, the “green dot” speed shown on PFD in Airbuses (and used as managed speed for climb) is $V_y$. And since this is usually preferred for climb, I am surprised a rather new Boeing wouldn't show it.

I find the normal climb speed on the B767 and B777 is pretty close to best rate.

@RalphJ exact numbers aside, my point here is that small GA aircraft also usually have max takeoff weights 30-60% percent higher than their empty weight. I'm not sure the weight range is much narrower in GA aircraft than in transport aircraft, like you state in your answer.

Well they do of sorts.   But your pilot friend is right, they are not tabulated for a transport category aircraft like is done for a Part 23 aircraft.  Due to the large payloads, CG ranges and configurations, the best rate of climb speed must be computed for each situation.  There is no one published Vy for a large transport category airplane.
Incidentally the best rate of climb varies for any aircraft you fly, it’s just that for smaller airplanes that range is quite small. As an example, in a Cessna 172 the best rate of climb Vy varies only a few knots between sea level and its service ceiling. Larger, more powerful GA aircraft will have tables published in the performance and limitations sections of their AFM listing this.  Saying that any airplane has a single best rate of climb speed is a bit of a misnomer, similar to saying that an airplane has a ‘stall speed’, etc.

Actually modern jets in general do have massive excess thrust. One big reason is due to safety requirements to handle engine loss. Vy is a big issue while on a single engine or piston props. With two turbofans/turboprops running and at low altitudes, they "climb like a rocket" compared with single piston and even twin pistons with big engines.
Additional reasons for the substantial thrust to weight is necessary acceleration for short runway takeoffs and just as importantly to still have excess thrust to climb way up there where the air is thin and high subsonic cruise can be achieved. Up there the airframe and the engine are both more efficient, but due to lost thrust with altitude, you need big powerful engines.
Remember that for jets and high performance turboprops, 3000-4000fpm climbs are common. It just doesn't take long until the terrain is no longer and issue and its more important to get to the destination faster and also save fuel. Flying on Vy typically wastes fuel, because you are running the engine at max continuous power/thrust, while not getting neither the best forward nor the best vertical speeds.
Big jets prefer to climb at speeds above 250KIAS, so they hit the 250KIAS below 10000ft limit.
But strictly speaking, what would be used as Vy, to clear critical terrain right after takeoff, something in the V2+20 to V2+50 range would be used. V2 might actually produce a better angle of climb, but its very uncomfortable to fly that close in case an engine quits. Even then this is used for as short a period as possible. V2+30 requires the aircraft has flaps deployed and is flying at half the speed it would like to go.

V2 is calculated before every takeoff. The calculation takes into account many issues including aircraft weight. So indeed "Vy varies."

@Marcelo Pacheco good answer, except "Vy wastes fuel...while not getting best vertical speed".  Vy is typically best time to climb, at an airspeed that is near both the minimum combined induced and parasitic drag, and the best thrust output, of a fixed prop/engine rpm, for a recreational aircraft.  It may have more to do with much wider weight variation.  A large transport aircraft may have a weight difference of 100 tons from one flight to another.


text:
Is there any engine that doesn't use a propellant to produce thrust?
As far as I know, all engines used in aircraft produce thrust by means of some propellant: kerosene, hydrogen peroxide etc. Maybe I've missed something, but could you tell me, are there any engines that can produce thrust without using exhaust gases expelled from the nozzles? Is it even possible?

Like an electric motor?

Would a straight piston engine whose exhaust doesn't provide thrust count?

Oh, thanks @RonBeyer, I didn't even know about such crazy thing like  NASA Helios. This is really amazing!

@Timpanus, no, a piston engine is not what I needed. In simple words, I just wanted to know about engines that produce thrust without fuel combustion being involved.

Your title is confusing to me then.  I consider "combustible fuel" and "a propellant" to be different things.

@Storix thanks for the clarification!

@BowlOfRed, Sorry, I'm a newbie to the aviation field. Could you give an example when propellant and combustible fuel are different things?

@Storix: A water bottle rocket uses water as propellant but doesn't combust fuel. A piston driven propeller uses no propellant (it propels the air instead) but burns fuel. An ion engine for spaceships uses xenon gas as propellant but burns uranium as nuclear fuel.

@slebetman, ok, I think, I got it. One more thing: are "a propellant" and "reaction mass" equal terms? For example, in the case of a water rocket, is it correct to say that water is a propellant and reaction mass which causes constant acceleration?

@mins, Yes, of course, the question concerns the aircraft engines only. I thought, it is assumed by default on the aviation stackexchange site :) Thanks for your suggestions, but now I'm thinking about posting a new more general question regarding the ways of producing thrust to make the aircrafts to fly. I feel I need more research on a subject :)

Re: "Maybe I've missed something, but could you tell me, are there any engines that can produce thrust without using exhaust gases expelled from the nozzles?"  Would turning around any exhaust stacks / pipes that may be present on a piston engine to point forward rather than having any rearward direction, satisfy your criteria?  Or are you asking about an engine that can work without emitting any form of exhaust whatsoever?

No matter which kind of engine, you need some way to supply the engine with energy and you need some way to store that energy until you need it in flight.
Short of going nuclear (and there are some nontrivial engineering problems involved in designing a halfway safe nuclear reactor that you can put on a plane and keep it lightweight enough that flying it isn't pointless), chemical fuels for burning have the best available ratios of weight to energy stored. This is particularly important for aviation because more weight means more induced drag which means more thrust needed.
Joule for joule, the efficiency of fuel-burning engines is not great, but the superior energy density of hydrocarbon fuel more than makes up for that and allows them to supply more useful thrust per kilogram of energy storage lifted than either batteries (low losses, but very heavy) or fuel cells (medium losses, but require small-molecule fuel such as hydrogen or methane that still have markedly worse energy densities than kerosene).

Since you speak about "exhaust gases" in the question, note that current aircraft derive at most a small fraction of their thrust from ejecting exhaust gases. Propeller planes don't generally make use of the exhaust at all, and "jet" engines are practically always high-bypass turbofan engines where most of the energy from the combustion goes to drive the fan that blows intake air backwards without burning anything in it.

Thank you, especially for the part about the exhaust gases! I see I had totally wrong idea about the creation of thrust. Things are more clear now.

"Short of going nuclear" Not entirely unheard of though

The Soviets did build a nuclear aircraft engine: the Kuznetsov NK-14 which was meant to be used on the nuclear powered TU-119. The plane was never completed due to cost constraints but the engine was built and ground tested.

Even flying a glider you need to get it up somehow.. well - lets see the paraglider :) Though the paraglider has no engine. Talking about the engines, today we always 'burn' something to get the energy out of it..

Even in a pure turbojet most of the air is just heated, but does not take part in any chemical reaction. The nitrogen, which makes up almost ⅘ of the air, is inert, and even of the oxygen just a small fraction is actually used to burn the fuel.

Yes there are. Although they are not for commercial aviation. One example are solar powered engines:

Source Wikipedia
Alternatively, there are other means of storing energy - rubber bands, but this one is just for model aitplanes.

Source: Pinterest
But in either case, you need store energy somewhere, and best way is to use oil-based fuels.

Big +1 for the rubber band airplane!

Wow, a radio controlled rubber band airplane? That's... wow!

@FreeMan I see where your sarcasm is coming from. Corrected :)

Some simple helicopter models like bamboo copter use rotary inertia as the power source. Either rotor or, in some cases, the whole simple model is spun up on the ground, providing enough rotating energy for the short flight with the help of the blades, same as for the real helicopter. However I am not aware of the full scale machines relying on such a power source.

To be strictly accurate, those are not helicopters, they are autogyros.  The terms have distinct meanings and all helicopters derive their power from engines.

It is autogyro when descending. When going up vertically, it is probably not.

The 'bamboo copter' uses 'rotary inertia' as power source only when climbing. When descending, it gets its energy from the column of air that traverses the rotor disk while losing altitude...

If you mean rocket engines and jet engines, excluding prop aircraft, there is of course the ion engine. It doesn't use "propellant" in the sense of heated gas thrown out the back, but it does exploit a stream of what we call "protons" for thrust.

Wrong. "Ionized xenon" is plasma, not gaseous. Plasma phenomena cannot be modeled or understood in terms of the behavior of gases.

Ah, I accidentally misread "protons" as "photons".

Might be worth linking to https://en.wikipedia.org/wiki/Ion-propelled_aircraft, and being clear you are not talking https://en.wikipedia.org/wiki/Ion_thruster which are notable useless in atmospheres.

@fertilizerspike You seem to be mixing up the two kinds of ion engines that work quite a bit differently.

Nope. I'm definitely not doing that.


text:
How far would an all electric A380 fly?
I have edited this to one question and thank you for all the feedback .
I am trying to understand if there are any theoretical improvements in range when applying electric propulsion to large commercial craft such as the A380, or if the theory favours smaller commercial turbine powered craft.
A good answer for a Cessna seems to be the specific capacity of the battery = range in nautical miles. However the weight of the fuel in a Cessna is under 10% of its MTOW, whereas an A380 tops 40%.
The second good answer asserts that you would design a new aircraft, you don't convert an existing one that has been optimised for chemical combustion at the point of propulsion. The proof here is provided by the Sun Flyer.
The third theory is that doesn't matter how big or small the craft is, it's a ratio of battery weight to MTOW.
Whether we decide to convert an aircraft or design a new one, suppose we had to make an all electric commercial aircraft with the greatest possible range, would you start big, such as with an A380 or start with the smallest aircraft and why?
Some A380 facts to use are here: https://web.archive.org/web/20161012152549/http://www.airbus.com/fileadmin/media_gallery/files/tech_data/jetFamily/media_object_file_A380_specifications.pdf
Maximum landing weight= 386 tonnes
Empty weight=282 tonnes
Typical payload=66 tonnes
Thanks for the careful thought on the replies already.

Well we can make a rough guess based on what is possible on smaller craft, and the result of my guess is that you could probably build a 380 that could fly for 30 minutes to an hour with existing battery technology.  Gonna be a while yet....

Possible duplicate of What energy density is required for the batteries in order to make an all-electric analogue of the Cessna 150 or similar plane?

You can make anything fly. Even a brick will fly. Whether it's worth even trying is another story. :)

This post computes that it requires 185 MW to get a B747 airborne. With a capacity of 10.6 MWh, I get a flight time of 3.4 minutes.

The A380 is a fascinating aircraft, but proportionally to make it electrical or to make this to any other aircraft the result will be proportionally the same. Generalizing the answer would be interesting.

You have crafted your conditions to assure that electric fails. I discuss this here.

Let's suppose we had to convert a commercial aircraft to an all electric craft tomorrow and use all the available parts, batteries, thrust devices and airframes becoming available, as well as take off and land on conventional runways, with a typical payload etc, would this thing fly?

This amount of specificity makes what would normally be pretty silly into something that can, if only just, work for a question.
The best aviation-specific electric engines are only somewhat heavier per unit weight (but much less powerful in absolute terms) than the best turbine engines, and will make up for it in using bus bars and distribution boards instead of extensive fuel piping, heating and pumps.
You could not simply stack a lot of small electric engines under the wings, as that would ruin the aerodynamics - the wing couldn't produce any lift due to fast jet air flowing under it. It would either have to be an all-custom design, or the engines would have to replace the turbines directly. Such engines are not currently available, but, with superconducting rotors and stators, are potentially achievable within the next decade.
Per calculations done here based on the Breguet equation, a perfect electric airliner will fly up to 10 nmi for each 1% of its weight that is dedicated to the battery. More precisely it's 9.9 nmi at a 0-100% discharge cycle with 100% efficient fans. A more realistic but still optimistic number is 7 nmi, with less than perfect fans and allowing for a 20%-95% cycle that accounts for degradation and emergency reserves.
At takeoff, the A380 can carry up to 254 metric tons of fuel, but that would only leave 44 tons for the payload, supporting up to 400 passengers, and its maximum landing weight would require less than 100 tons of fuel aboard.
Assuming the hypothetical electric 380 is reinforced for a MLW equal to MTOW (575 tons), and counting on savings from general modernization and removing the fuel system to absorb that reinforcement weight, a more realistic battery weight is 230 tons, allowing for a 68-ton payload, supporting the usual 600-passenger capacity. This makes for a 40% battery weight, and thus approximately 280 nmi or 520 km operational range.
So the short answer is: if electric engines with similar power and size to the Trent 900's cores can be produced, the airplane will fly, but its operational range would be just ~500 km, less than 4% of what the current A380 is capable of.
Practically, you would not use an A380 to fly short distances, due to its complexity in ground handling of the two flight decks. There was the Boeing 747SR, built just for the Japanese domestic market, to save on airport slots. The largest plane you'd use has to be something versatile and quick to load and unload, such as the A350 XWB.
But there are short routes for small aircraft 1that are simply begging to be electrified, and if the legacy factor and lack of infrastructure don't interfere, they likely will.

you're missing to factor in that those best electric engines produce a fraction of the power of the engines installed on an A380, so you'd need many more of them. Which makes the equation even less appealing, as it'd likely run out of electricity before even reaching the runway.

@jwenting This is under the assumption that all-superconducting engines that can replace the Trent 900's cores within a similar space envelope can be manufactured. It's difficult, but I'd rate it as short of impossible.

That'd depend heavily on the development of cheap high temperature superconductors, something we're far short of. Theoretically possible, but fantastically expensive.

@jwenting "Cheap" was never part of the equation. I'm aware of the development of helium+nitrogen cooled SC-rotor, SC-stator engines based entirely on current HTSC technology. Difficult, but it might be possible. And the gap to cover isn't that wide. The Emrax is a simple glycol-cooled copper-wound motor - it's just scalability that HTSC need to address.

Considering photovoltaic effect it is totally insufficient due to the ratio of skin area to Weight
Using the wings to be filled with batteries cells is non sens for maintenance.
Continued on next comment

So let us fill the cargo volume with batteries cells and let us replace the engines with electrical motors driven propellers
Comparing the matters to what happens with electrical cars, my comment would concern every commercial aircraft not specifically the A380.
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Assuming it flies with no passengers, and considering similar power efficiency as on cars, the aircraft will fly about 1/5 its nominal range with fully charged batteries.
On the next flight with 60% quick charge, it will fly at best 12% its nominal range

"On the next flight with 60% quick charge" That right there is one of the biggest stoppers of electric commercial aircraft, just as it is for electric cars. The infrastructure to replace the "gas station" (whether a building for cars, or a tank/pump for aircraft) with a "quick charge" or "quick change" for replacing spent batteries with fully charged ones will take ages to build out. Even if/when the battery capacity and HTSC electric motors of sufficient capacity are developed.

@FreeMan Keep in mind we're talking sub-1000 km routes even in the best-case scenario. Strictly short routes. No way to get past that with any currently expected advances.


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Shock wave on airfoil
I was wondering the other day, during the trans sonic regime, when the velocity over the airfoil becomes supersonic,why does the shock wave standing on the airfoil moves downstream when the free stream velocity is further increased?

With a higher flight velocity (still subsonic), the supersonic pocket on the upper side of the wing starts earlier and takes longer until it collapses into the shock. That is quite normal, as the speed over the whole upper wing is increased with increasing flight speed.
Think of what happens at the extreme case of accelerating flight speed towards Mach 1: Now the shock will move towards the trailing edge with increasing Mach number and will stay there once the flight speed is above Mach 1 (NOTE: This is valid only for straight wings; correct the Mach number upwards for sweep effects).
Things become nasty when the shock interacts with lift creation and starts to fluctuate back and forth - this creates a buzzing sound and might even cause flutter. The mechanism is that the stronger shock causes flow separation and lift loss, which in turn shrinks the shock, which allows lift to recover so the shock grows again. With the growth, the shock moves back and jumps forward when it becomes weaker. This back-and-forth jumping of the shock creates not only a fluctuation of lift, but also of pitching moment.


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Why are no HAPS (High Altitude Platform Station) solar planes of a biplane design?
HAPS are fragile due to weight constaints, e.g. NASA’s Helios. Biplanes are more robust.
Although biplanes have high drag, HAPS only need to move slowly because at their operating height the wind speed is less than 50 mph, and to maintain station they only need to overcome that.

Possible duplicate of Can biplane or triplane designs be revived with modern materials?

I don't think this is a dupe. It has nothing to do with the construction materials available today vs in 1919 (with the exception of solar panels). It appears to be asking why the biplane format is not used for this category of aircraft.

That said, there would be a considerable loss of sun-facing wing surface by shortening the wing and placing half of it (roughly) in the shade below the other half, so that may be sufficient reason to not use a biplane design.

HAPS are fragile due to weight constaints. Biplanes are more robust > Robustness is heavy and biplanes are aerodynamically inefficient.

A biplane is more robust than a monoplane per unit weight. This is because the monoplane supporting beam has to live inside the max vertical depth of the wing. With a biplane the beam can be as deep as the vertical distance between the upper and lower wing. The biplane has a worse lift to drag ratio than equiv monoplane. But drag increases dramatically with air speed. As the HAPS is only slow flying maybe the drag is acceptable. At high altitude the Reynolds number decreases, this raises stall speed. Stall speed goes down with lower wing loading (plane weight / wing area). Biplane wing loading is lower so will have a lower stall speed. Of course it's all trade offs. These are arguments for the biplane.

I've just realised recreational gliders have most of the same constraints, and there are no biplane gliders.


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Commercial aircraft minimum and maximum climb angle and rate?
Is it possible for a commercial pilot to set a minimal rate of climb - for example, 350' NM climb rate, target heading of 200 degrees and target altitude of 2000 feet above MSL - before getting radar vectors from ATC when they hit the target altitude?

Minimum climb angle & rate is zero, but that's a pretty bad idea on most departures.  Maximum rate/angle depends on a large number of variables: model of 737, aircraft weight, temperature, derate or assumed temperature used for takeoff, pilot technique, use of "improved climb" procedure or not, and probably a few more.  If you got the very best of all of the above, the climb rate would be pretty spectacular, but nothing you could count on seeing routinely. Lots & lots of "it all depends" here.

Thanks, trying to keep this in layman terms - so for example if there was good visibility, no wind, 737 Next Generation -800, passenger load of 160, about 160,000 lbs, average temperature and derate, using GPS.

You are asking several different questions, but I can answer the first part easily enough.
First, let's distinguish between climb rate and climb gradient:  climb rate is expressed as feet per minute, climb gradient is expressed in feet per nautical mile.
The autopilot I am familiar with can be programmed for a target climb or descent rate, heading and altitude.  With ground speed and some math a FPM rate could be set that will achieve a desired gradient, but I am not aware of any systems where this can be actually set.
737 takeoff and climb performance will vary greatly depending on weight and other variables.


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Can supersonic conditions be replicated on the ground?
In a previous question the P&W J 58 engine for the SR-71 was discussed. The engine was a new and innovative design combining the characteristics of a turbojet and a ramjet.
Before flying with such a new design I would think they would want to test it on the ground as much as possible to ensure the safety of the pilot and the aircraft. Otherwise you're sending a guy up with things that should work but so far only on paper. (I know that's what test pilots signed up for but they don't want to be reckless about it.)

J-58 ground test. from Wikimedia commons
From what I know a pure ramjet won't even operate at zero airspeed. What I'm wondering is how much can be learned from ground testing on such a radical new design, or any supersonic design for that matter? Can supersonic conditions be replicated on the ground?

There are supersonic cars which operate on the ground. But I suppose you are talking about testing engines on the ground? And the J-58 could operate on the ground because it could do subsonic operations too (takeoffs, landings, and refuelings).

Yes, supersonic conditions can indeed be replicated on the ground for purposes of airframe and engine testing.
Arnold Engineering Development Complex (AEDC)
The U.S. Air Force and NASA both operate supersonic wind tunnels, as do Lockheed, P&W, and others. The most advanced of these facilities are probably those at Arnold Engineering Development Complex on Arnold Air Force Base. I won't mention any specific numbers from memory because they've likely changed since I worked there and I can't remember exactly which ones were classified/FOUO/otherwise-not-public, but let's just say they're pretty good.
AEDC has test cells designed both for testing jet engines as well as for testing scale models of airframes (or full scale airframes if it's a small enough airframe, such as a missile.) Additionally, it has facilities for testing upper-stage rocket motors. Lower-stage rocket motors don't need the facilities at Arnold, as those are designed to operate in the atmosphere and can just be lit up out in a desert somewhere.
According to Wikipedia (which I believe is sourced from publically-available information,) Hypervelocity Wind Tunnel 9 can generate air flow of Mach 14 with simulated altitudes from sea level to 173,000 ft.

AEDC Tunnel 9 (with test article)  Source: Wikipedia
In case that's not enough, G Range can fire projectiles at roughly the speed of low Earth orbit. However, this one is kind of stretching the definition of "supersonic," since it's typically pumped down to such negligible air pressure that Mach number isn't really defined before a shot at that kind of velocity. Still, as the wiki article indicates, it's capable of shots at up to 1.7 atmospheres, so it definitely counts as legitimately supersonic on those shots.

AEDC Range G  Source: Wikipedia
Holloman Air Force Base
In addition to wind tunnels and other facilities at AEDC, the U.S. Air Force also occasionally uses rocket sleds, such as the one at Holloman Air Force Base, which currently holds the speed record for any type of non-air/space vehicle at 6,416 mph / 10,326 km/h (Mach 8.5.)

Holloman High Speed Test Track Source: Wikipedia

Land Speed Record Rocket Sled at Holloman Source: Wikipedia

The "Speed Limit" at Holloman Air Force Base
Source: some guy's website, which got them from publically-released images from Holloman AFB

To be fair, that sled ran inside a helium tube (okay, some thin plastic sheeting draped over a stick frame - it was destroyed as the sled passed through it - but still an environment in which a jet engine would have some trouble) and the front-wheel hardware was almost completely ablated away at the end of the run. Not one of your more practical test beds, really, but 'twas fun to watch.

A) You worked there? That had to be fun! B) I'm sure that those silver blocks at about 5 o'clock and 10 o'clock in the first picture are for a variety of sensors, but I'd have to imagine that their rather blunt leading edges would have some sort of impact on airflow at Mach 14!!! Sheesh!

@FreeMan It was fun. It was during college. I considered taking a permanent position there after college, but I got another offer that worked out better. I still have some friends who work there, though. I haven't been to Tunnel 9, though. That one is in Maryland. I worked at the main base in Tennessee. I'm sure you're right that those boxes produce some interesting aerodynamic effects, but they're downwind of the test article, so it shouldn't really matter much.

An extra +1 for the speed limit sign! Dang, that didn't work... :(

@FreeMan I originally saw it on a t-shirt and was pleasantly surprised to find that the sign was actually there. :) Good luck pulling over the guy violating it, though. Also, good luck violating it.

My favorite episodes of Mythbusters were the ones that they used the rocket sled. It's just so awesome to watch something move that fast. Especially when they added a second stage. Just when you think nothing could go that fast the second stage kicks in and... DAMN!

It is a struggle, but it can be done. For the J58, Pratt&Whitney had a test facility at West Palm Beach which would simulate conditions at speed and altitude. From this site:

The picture actually shows test cell A-1 at Pratt and Whitney’s West
  Palm Beach facility. […]. This test cell was actually an altitude simulation
  cell used for testing purposes. (We had a additional cell used for sea
  level runs for motors which were overhauled onsite also.)  It used a
  non-afterburning J-79 as a slave motor. The exhaust of the slave was
  introduced to the inlet of the J-58 through a series of valves thereby
  simulating the speed, temperature, and density of the air at the inlet
  normally seen during flight. In this particular picture the motor is
  running at sea level as indicated by the inlet screen.

This answer inadvertently becomes the answer to this question.

Also, I just realized that the orange section at the end of that engine does not appear to be paint, but glowing hot metal! Is that by design? I realize that it's mighty hot at that end of the burner, and that we normally don't see that because of the nacelle, but does the outer part of the exhaust nozzle normally get that hot?

@FreeMan I had the same thought, and I wondered if normally it would be more cooled by airflow if it were actually in flight.

As shown here, Lockheed Martin has a supersonic wind tunnel capable of generating wind speeds of up to mach 5. A quick Internet search revealed an extensive list of other supersonic wind tunnels, indicating that such tech is fairly widespread. In addition, as suggested by SMSvondertann, jet- and rocket powered cars such as the Thrust SSC have attained supersonic speeds on land.

My first thought was, "but was it open when they were developing the SR71?". The answer to that is, yes - it was opened in 1958!

An important caveat is that high mach wind tunnels can only operate for short times (they can't accelerate the air that fast in real time, and instead function by filling a huge pressure building and then venting it into the tunnel).  LockMart's tunnel is rated for 15-110 seconds at mach 5.0-0.3.  It gets even worse at hypersonic speeds; wind tunnels can only hit those levels for at most a second or two meaning that R&D needs to involve using rockets to accelerate physical hardware to verify the CFD simulations in flight.

@DanNeely Maybe Lockheed's tunnels have that restriction, but AEDC's tunnels don't. They can (and do) run for hours on end. They also have to call TVA about every half hour to tell them how much electricity they're planning to use when the cells are running. :) Otherwise, they'd literally overload or underload the local power grid.

Not quite on the ground, but some organisations go to quite extreme lengths to simulate supersonic conditions. Take NASA and their supersonic parachute intended for Martian atmospheric entry, also known as the Low-Density Supersonic Decelerator. They recently tested this on a quite startling rig:
-The parachute was dropped from a helicopter
-A 1km steel rope attached the chute to a winch via a pulley
-The winch keeps the rope under tension...
-...and is attached to the the star of the show - a 4-motor rocket sled, designed to accelerate the parachute to extreme speed before it hits the ground.
This rig is worth mentioning not least because the description conjures up an image of a pile of empty ACME boxes!
See here for further information

..."meep, meep!"

@TomMcW that road runner doesn't stand a chance ;)

@TomMcW   And so it begins ...

The link has gone stale, but NASA's 404 page is cute enough that I won't instantly fix the link.


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What is the reason for circulation of air around an airfoil? Based upon the circulation of air, why are airfoils shaped the way they are?
Foreword: I am an Aerospace Engineering student and next year is my third and final year, starting September 2019.
I was taught approximately 2 semesters ago that the correct theory of lift is that airflow circulates around the wing and so the upstream increases the velocity on the top surface while reducing the pressure, and reduces the velocity on the lower surface while increasing the pressure, which causes a difference in pressure that generates lift. Prior to that, I had the incorrect, yet simpler 'equal transit time' theory, which I'm guessing is taught at all universities at the early stage due to the simplicity of it. My problem is that I still base a lot of things on incorrect theories of the production of lift.

What causes air to circulate around a wing, or even a cylinder? My answer to that would be due to viscosity and boundary layer effects, is that right or wrong? I'm honestly unsure.
Since this theory was first being explored by analysing a cylinder, and it was determined that a cylinder can produce lift due to circulation of air (and the direction of upstream air), why do we have airfoils shaped in the way they are? I have many answers to this question which now I'm honestly unsure of due to the mix of theories in my head and some are based upon incorrect lift production concepts. Please give me a detailed answer for this question especially.

As a general comment, keep in mind that theories about lift are not mutually exclusive (except the equal transit time, that's just wrong). Splitting flow in translation and circulation is mathematically appealing but not necessarily good for an intuitive understanding.

I can't believe they are leaving out the whole acceleration of mass aspect which is what most "lift" is.  Stand behind a propeller, or under a helicopter's rotor, which are both just wings going in a circle, in the blast of the accelerated air mass, and it's kind of hard to miss.

John K, by accelerated mass are you trying to say that upper surface particles accelerates more due to the longer distance covered and it must meet with lower surface particles? If that is what you are saying, then it is wrong.. but please do elaborate on what you mean.

@AliAlKayyali what I am saying is that the wing induces a large package of air, some below but mostly above, the wing to move downward as the result of the wing passing through it.The circulation and pressure distributions are all a part of that process. A flat sheet of plywood at a positive angle will produce lift just on the air deflected by the bottom of the sheet, but an airfoil influences a large part of the air above it as well to also move down, so it's way more efficient at doing it. When u stand behind a prop you are getting blown by the accelerated mass made by the "revolving" wing.

@AliAlKayyali Since Newton's Law is a factor here, I suggest you google some word strings like "newton airfoil lift" or similar and you will find articles that explain the action/reaction aspect to lift generation.

@JohnK okay so your point is that the downwash at the trailing edge is what causes lift using Newton's third law? So can I combine theories, such as the theory of momentum and the theory of flow circulation to get a correct explanation of lift and why airfoils are designed the way they are? Cause for me it now makes everything clearer with the theory of momentum. I mean what really matters is how to obtain lift and how to improve it (mathematically) but I still also rather have a good intuitive understanding of the correct theory.

@JohnK Also, can you explain to me the reason why air circulates around a wing? Does a cylinder have to be spinning in order for the air to circulate around it, and why?

Yes the cylinder has to spin so that its surface friction can induce circulation by making the boundary layer want to follow the moving surface of the cylinder, otherwise the air just goes around the top and bottom equally and rejoins aft somewhere.  Try this out: http://www.onemetre.net/design/downwash/Circul/Circul.htm

I'm going to suggest this is duplicate of https://aviation.stackexchange.com/questions/16193/how-do-wings-generate-lift/, which has a pretty good, and correct, explanation, in the accepted answer. But since you mention circulation specifically, you may also want to refer to https://aviation.stackexchange.com/questions/47251/what-is-the-physical-meaning-of-circulation-found-in-kutta-condition.

@JanHudec thank you very much, that second link was very helpful. I can now conclude that a low RN causes circulation, and it is typically for a wing or some body with a sharp trailing edge? Though just one more comment, was the Kutta Condition determined experimentally? Because I can't find any mathematical model to represent it.. it seems as though it is only useful in solving the Kutta-Joukowski theorem. The reason I want to look more into the theory of circulation is just to widen my vision in the field of 2D and 3D flow over an airfoil. I am planning to base my M.Sc. thesis on this.

@AliAlKayyali, yes, Kutta condition, and Kutta-Joukowski theorem were determined experimentally, and you need some experimentally determined values to calculate anything with them. It is possible to model the fluid flow mathematically using the Navier–Stokes equations, but those don't have analytical solution for any non-trivial case, so you need computer program to calculate them. For the simple 2-D case there is the famous XFOIL.